1. Field of the Invention
The present invention relates to an imaging system including a receiver sheet which is capable of providing high quality visible images through reaction with an electron donating color precursor. More particularly, the present invention provides a receiver sheet for use in an imaging system wherein the color precursor is transferred to the receiver sheet in a polymerizable activated ethylenically unsaturated material and wherein the receiver sheet includes an amine salt capable of polymerizing or otherwise sequestering excess polymerizable material by a Michael addition to convert the polymerizable material to a more desirable form.
2. Description of the Prior Art
Commonly assigned U.S. Pat. No. 4,399,209 discloses a transfer imaging system. The system comprises an imaging sheet comprising a first support having a coating on the surface thereof. The coating typically includes a photohardenable composition which is encapsulated with a color precursor. Images are formed by image-wise exposure of the microcapsule coating to actinic radiation.
After image-wise exposure, the microcapsules are ruptured. The viscosity of the photohardenable composition increases substantially upon exposure to actinic radiation through mechanisms such as cross-linking and simple polymerization. Therefore, when the microcapsules are ruptured, the photohardenable composition which received a strong exposure will flow very little if at all while the unexposed or weakly exposed photohardenable composition can flow relatively freely. In practice, the photohardenable composition typically comprises an ethyenically unsaturated compound, and more specifically an activated compound, typically a acrylate.
After image-wise exposure and microcapsule rupture, the photosensitive composition including the acrylate and the color precursor migrate to a receiver sheet which comprises a second support having a layer of developer material on the surface thereof. The color precursor reacts image-wise with the developer material to form a color in the form of the desired image.
The transfer of the acrylate to the receiver sheet can pose one or more problems. A first problem is that the acrylate may diffuse into and across the imaging sheet and blur and thereby reduce the intensity and clarity of the final image. A second problem is that a small amount of the acrylate monomer may remain on the surface of the receiver sheet. This is undesirable because some individuals are sensitive to acrylates.
Attempts have been made in the art to cure the deficiencies associated with acrylate transfer. Commonly assigned U.S. Pat. No. 4,483,912 teaches that the image diffusion problem can be reduced by including a curing agent in the microcapsule coating which reacts with the photohardenable composition released from the microcapsules and cures or otherwise hardens it. The curing agent is contained in the same microcapsules as the photohardenable composition or in separate photoinert microcapsules. The curing agent is typically a thermal initiator in that upon heating the agent generates free radicals to initiate polymerization of the released monomer.
Similarly, U.S. Pat. No. 4,784,932, assigned to Fuji Photo Film Co. teaches that the image diffusion problem can be reduced by including a curing agent in the developer material. As in the case with U.S. Pat. No. 4,483,912, the curing agent generates free radicals upon exposure to heat.
U.K. Published Specification No. 2,203,851 discloses a picture receptive material having a substrate, developer material and a hardener which can polymerize a polymerizable compound without the need for external energy. The hardening material is a compound having two or more nucleophilic groups in the molecule, and preferably a compound with functional imino, amino or sulfino groups. Although the additive materials according to this reference can function to polymerize the released monomer, the image quality produced is significantly reduced when the additives are used in association with acidic developer materials. More particularly, the basic nature of the amine and imine additive materials can cause them to prematurely react with a phenolic resin. This premature reaction can inhibit the reaction between the developer material and color precursor, which is typically a basic material.
Accordingly, there continues to be a need in the art for a receiver sheet to be used in association with acrylate imaging systems wherein the receiver sheet can develop high quality images and polymerize released acrylic monomeric material without requiring additional radiation exposure steps.